1. Field of the Invention
This invention relates to an antenna device to be used mainly for portable telephone equipment.
2. Description of the Prior Art
Recently, antenna devices to be used for portable telephone equipment have been attracting great importance to be provided with a performance of preventing the receiving strength from being degraded due to change in application environment. For this, a conventional antenna device comprises a first linear antenna for producing an electric current flux and a second linear antenna for producing an electric current flux which are disposed perpendicularly with respect to each other. With the antenna device arranged as above, the operation will be explained below.
First, both the first linear antenna and the second linear antenna receive a radiowave. With conventional portable telephone equipment, degradation in the receiving strength characteristics due to a change in application environment will take place, particularly when the telephone is used while being handheld, the receiving strength characteristic is degraded due to influences of adjacent bodies including human bodies and/or a deviation of the setting angle of the device from the horizontal level.
In order to overcome these disadvantages, an improvement has been made in that the second linear antenna has its antenna axis disposed perpendicular to that of the first linear antenna to produce different radiowave environments by the respective antennas, and the outputs of two receivers are monitored to switch them to the antenna which is in the better receiving condition. This is called polarization diversity (for example, refer to: T. Utano; "Propagation Characteristics of Polarization Diversity Reception at Base Locations for Mobile Radio", IEICE, vol. J63-B. No. 5, PP. 540-541, May 1980).
With the conventional antenna device as such, all plural antennas to be used are linear antennas having the same basic operational principle, so that they will be degraded in characteristics by any adjacent bodies including human bodies in the same manner, which means that a problem has arisen in that two antennas to be used are simultaneously degraded in characteristics, so that sufficient diversity effect cannot be obtained.
In addition, the first and second antennas are set so as to have their antenna axes disposed perpendicular to each other, which means that they must be disposed into two different directions, thus making miniaturization of a device disadvantageously difficult. In addition, with an arrangement as shown above, such a problem has been pointed out in that the linear antenna having its antenna axis disposed horizontally does not have a horizontal plane omni-directivity.